Recent advances in bipolar/BiCMOS IC design and fabrication make possible the integration of digital and analog circuits on the same IC chip. This technology is widely used in mobile communication systems where a digital core is combined with analog RF circuits. The digital and analog circuits are typically formed as a variety of components near one surface of a wafer. These components may be at several levels separated by inter-metal dielectric layers. Usually, the topmost layer is made of a dielectric material and serves as a passivation layer for the entire structure.
The integration of the digital and analog circuits causes noise coupling between digital and analog circuits. Analog circuits are especially affected by the noise generated in digital circuits. This significantly limits the performance achieved in analog signal processing and data conversion circuits, such as differential amplifiers that are extremely sensitive to the noise at the differential inputs. FIG. 1 illustrates a noise path between digital and analog circuits, wherein region 4 is a digital circuit region and region 6 is an analog circuit region. Arrows 8, 10 and 12 symbolize one of the noise paths in the substrate 2.
Besides the noise interference between digital circuits and analog circuits, noise interference also exists between digital circuit components.
There is a significant dependence of the noise coupling through the substrate on the constitution of the silicon substrate. Various methods have been developed to break the noise path in the silicon substrate. One commonly used method is forming isolation layers in the substrate. As shown in FIG. 1, an isolation layer 14 breaks the noise path between circuit regions 4 and 6. Isolation layer 14 is typically formed of dielectric materials. One example of isolation layer 14 is a trench isolation between the circuits to be isolated. To form deep trench isolations, trenches with nearly vertical sides are etched between the circuits and then filled with dielectric materials.
However, even deep trench isolation is not fully satisfactory when a full isolation between the circuits is required. This is particularly true when high-speed analog circuits are involved.
Another known method is placing a guard ring in the substrate and between the circuits to be isolated. As illustrated in FIG. 2, a p+ guard ring 20 is formed in a p− substrate 2. The guard ring 20 is grounded at node 22, thus creating a low resistivity path for the substrate noise. A noise current is more likely to take the low resistivity path to the guard ring 20 than a higher resistivity path to another circuit region.
FIG. 3 illustrates yet another method. A noise signal source 24 is connected to a device region 32 surrounded by N-well regions 28 and deep N-well region 30. The noise signal is sensed from node 26, which is physically connected to a device region 34. Interconnected N-well regions 28 and deep N-well region 30, when grounded, form a low impedance path for noise. A noise current generated at signal source 24 is thus conducted to ground, and noise signal sensed at node 26 is significantly lower. With such a noise isolation structure, if one circuit is formed in device region 32 and another circuit is formed in device region 34, the noise coupling is significantly reduced. Research indicates that the noise coupling level represented by insertion loss parameter S21 is about −60 dB at an operation frequency of about 1 GHz.
The previously discussed methods are effective for noise isolation. However, when the die size of the integrated circuit, which mostly consists of analog and digital blocks, becomes smaller in advanced technologies, and/or the frequency increases to over about 1 GHz, noise interference becomes more severe and better isolation techniques are needed.